This invention relates to integrated circuit oscillators using CMOS processes. More specifically, this invention relates to bias circuits to stabilize oscillation in ring oscillators and to oscillators that generate output frequencies that remain stable independent of process, temperature, and supply voltage variations. This invention also relates to methods to stabilize oscillation in ring oscillators.
Oscillators are used for timing controls in various applications. Usually, crystal based oscillators are used where frequency accuracy is very critical. Non-crystal oscillators show huge variations in frequency due to variations in device parameters with temperature, process, and supply voltage. There is a need for non-crystal, standalone oscillators with a fairly good level of frequency stability where the conditions are not so stringent as to necessitate the use of crystal based oscillators.
For example, K. Sundaresan, P. E. Allen, and F. Ayazi, “Process and Temperature Compensation in a 7-MHz CMOS Clock Oscillator”, IEEE J. Solid-State Circuits, vol. 41, no. 2, pp. 433-442, February 2006, and Yang-Shyung Shyu and Jiin-Chuan Wu, “A process and temperature compensated ring oscillator”, the First IEEE Asia Pacific Conference on ASICs, 1999 (AP-ASIC '99), pp. 283-286, which are incorporated by reference in their entireties, make use of bandgap reference circuits and other BJT based compensation circuits to compensate for oscillator frequency variations. Such BJT based compensation circuits are not only large in area but also involve complex processing steps which make them unfeasible in simple CMOS processes. A reference clock is also used in some cases to stabilize the output frequency. Further, these compensation circuits require precise tuning of various parameters and/or do not ensure sufficiently high frequency stability.